Some conventional microwave heating apparatuses of this type have been adapted to include an oscillation portion constituted by semiconductor devices, a plurality of amplification portions for amplifying the output of the oscillation portion, a heating room which is supplied with microwave electric powers outputted from the amplification portions through feeding portions, and an impedance detection portion for detecting the impedances of the feeding portions for supplying microwave electric powers to the heating room (refer to Patent Literature 1, for example). Such conventional microwave heating apparatuses have been adapted to control the oscillating frequency based on the result of detection by the impedance detection portion, to thereby perform stable cooking on objects to be heated within the heating room without inducing heating unevenness.
Also, there have been some conventional microwave heating apparatuses adapted to include a microwave heating power source which is variable in oscillating frequency, an antenna serving as a feeding portion for supplying microwave electric power to a heating room, and a detector for detecting reflected electric power from the antenna (refer to Patent Literature 2, for example). The conventional microwave heating apparatuses have been adapted to track an oscillating frequency of the microwave heating power source which minimizes the reflected electric power and to drive the microwave heating power source around this oscillating frequency, to thereby continuously drive the microwave heating power source with higher electric power efficiency.
Further, there have been also some conventional microwave heating apparatuses adapted to include an oscillation portion constituted by semiconductor devices, a dividing portion for dividing the output of the oscillation portion into a plurality of parts, a plurality of amplification portions for amplifying the respective outputs resulted from the division, and a synthesizing portion for synthesizing the outputs of the amplification portions (refer to Patent Literature 3, for example). Such conventional microwave heating apparatuses have been provided with a phase shifter between the dividing portion and the amplification portions. These conventional microwave heating apparatuses have been adapted to control the phase shifter for arbitrarily and immediately changing the phase difference and the ratio between electric powers radiated from the plurality of antennas, in order to perform uniform heating on objects to be heated within the heating room.
PLT 1: Unexamined Japanese Patent Publication No. S59-165399
PLT 2: Examined Japanese Patent Publication No. S62-048354
PLT 3: Unexamined Japanese Patent Publication No. S56-132793